1. Field of the Invention
This invention relates generally to digital communications systems, and more specifically to such systems that provide electrical power over digital data conductors in such systems, and more specifically to monitoring isolation between conductors in such systems and earth ground.
2. Description of the Related Art
Digital data communications systems can be made capable of delivering power over the same conductors that communicate the digital data. For example, the IEEE 802.3af Power-Over-Ethernet (POE) standard defines a way of delivering DC power in an Ethernet network that provides 10/100/1000 Mbps data communication. A POE link allows a Powered Device to draw up to approximately 13 W from Power Sourcing Equipment. A POE link requires high electrical isolation between conductors of the POE system and earth ground. This isolation requirement in 802.3af systems is vital to prevent life threatening conditions related to electrocution or fire hazards at an installation. Validation of a POE systems isolation conformance over the life of an installation becomes a significant issue. Maintenance of such conformance is presently a long term commitment to manual continuity verification by the user.
Briefly, a POE system is comprised of Power Source Equipment (PSE), cabling infrastructure, midpoint devices, patch panels, and end Powered Devices (PD). Loss of isolation to earth ground at any of these points will compromise total system isolation. There are a multitude of events that could cause the loss of isolation at each of these system points. It is possible that an installation may actually be installed with partial or no electrical isolation between its floating 48V system and earth ground. In most cases, a system still continues to function with an isolation loss. Erroneous manual verification in such systems could easily result in long-term, sustained isolation loss.
When a system has lost isolation, a secondary event anywhere in that system can result in an electrocution hazard in some cases a fire hazard.
The isolation requirement of the PSE 48V circuit precludes the use of many known circuits for measuring DC leakage, since such measurement itself may bring the system leakage out of specification compliance. As POE becomes embedded in LAN switches and thus finds widespread usage, the probability of life threatening events increases. Additionally, the power and/or voltage available to each PD is also increasing, from the standard 15 W to 30 W, further increasing the danger of isolation loss.
An apparatus and method for measuring isolation loss in a digital communications system that also provides power, such as a POE system, without the measurement circuitry or method significantly degrading isolation, is therefore desirable.